For most in vivo nuclear magnetic resonance (NMR) applications, enhanced sensitivity can be achieved by using a surface coil receiver (Ackerman, J. J. H., Grove, T. H., Wong, G. G., Gadian, D. G., Radda, G. K. (1980) Mapping of metabolites in whole animals by .sup.31 P NMR using surface coils. Nature (London) 283:167-170). Because detected signal intensity is proportional to the magnitude of the surface coil magnetic field, B.sub.1, which decreases with distance from the coil, the sensitive volume is confined to a hemispherical region immediately adjacent to the plane of the coil. Detectable sample noise (and signal) is restricted to the coil sensitive volume, and for magnetic resonance imaging (MRI) and spectroscopy (MRS) of localized regions, a surface coil receiver provides optimal sensitivity.
The B.sub.1 inhomogeneity of surface coils limits their utility as transmitters of many pulse sequences used routinely for MRI and MRS. When surface coils are used to transmit square or conventional amplitude modulated pulses, a continuous range of flip angles, .THETA., is produced throughout the coil sensitive volume since .THETA.=.gamma.B.sub.1 T.sub.p, where .gamma. is the gyromagnetic ratio and T.sub.p is the pulse length. Because only transverse components of the magnetization can be detected by a receiver coil (i.e., signal is proportional to B.sub.1 sin .THETA.), the signal intensity generated by a conventional pulse is spatially dependent. Consequently, no signal can arise from sample regions where .THETA. is a multiple of 180.degree., and in MRS applications, signals generated from regions where .THETA.=90.degree. may be partially canceled by signals produced where .THETA.=270.degree..
Conventional pulses can be used for single surface coil MRI, but in such applications the flip angle and T1-weighting is nonuniform over the active volume of the coil; consequently, the image quality is severely degraded.
For many applications (e.g., spin-echo MRI and spectroscopic imaging), circumscribing whole-head or whole-body coils with relatively homogeneous B.sub.1 are employed to transmit pulses, while a separate surface coil is used for signal detection. The complexity of the RF probe needed to conduct combined imaging and multinuclear spectroscopic studies using surface coil receivers can make these types of investigations impractical.
A much more serious problem in such applications is sensitivity. It has been demonstrated that the spectroscopic sensitivity of the double coil RF probe configuration (i.e., homogeneous B.sub.1 transmission, surface coil reception) is not increased significantly relative that obtained with single surface coil RF transmission and reception (Crowley, M. G., Evelhoch, J. L., Ackerman, J. J. H. (1985) The surface-coil NMR receiver in the presence of homogeneous B.sub.1 excitation. J. Magn. Reson. 64:20-31). Unlike the homogeneous B.sub.1 produced by a circumscribing coil, the direction of the B.sub.1 vector produced by a surface coil is spatially dependent, and consequently, will not have a constant phase relationship with the B.sub.1 vector of the homogeneous coil. This variation in phase over space will cause incoherent addition of the signal from the different parts of the sample (Crowley, M. G., et al., see above reference). This problem is less significant for multidimensional spectroscopic imaging (SI) where the total signal is subdivided into small volume elements (voxels) which individually contain reduced phase variation. However, for applications in which spectroscopic imaging is not needed, a 250% increase in signal-to-noise ratio (S/N) may be gained if the losses due to phase mismatch of nonequivalent coils can be overcome (Crowley, et al).
One approach to gain this increased S/N involves the use of RF pulses which induce uniform flip angles despite large variations in B.sub.1 magnitude and can therefore be transmitted with a surface coil. Recently, amplitude and frequency/phase modulated pulses have been described (Silver, M. S., Joseph, R. I., Hoult, D. I. (1984) Highly selective .pi./2 and .pi. pulse generation. J. Magn. Reson. 59:347-351; Baum, J., Tycko, R., Pines, A. (1985) Broadband and adiabatic inversion of a two-level system by phase-modulated pulses. Phys. Rev. A 32:3435-3447; Hardy, C. J., Edelstein, W. A., Vatis, D. (1986) Efficient adiabatic fast passage for NMR population inversion in the presence of radiofrequency field inhomogeneity and frequency offset. J. Magn. Reson. 66:470.482; Bendall, M. R. and Pegg, D. T. (1986) Uniform sample excitation with surface coils for in vivo spectroscopy by adiabatic rapid half passage. J. Magn. Reson. 67:376.381; Ugurbil, K., Garwood, M. and Bendall, M. R. (1987) Amplitude and frequency modulated pulses to achieve 90.degree. plane rotations with inhomogeneous B.sub.1 fields. J. Magn. Reson. 72:177-185; Bendall, M. R., Garwood, M. Ugurbil, K. and Pegg, D. T. (1987) Adiabatic refocusing pulse which compensates for variable RF power and off resonance effects. Magn. Reson. Med. 4:493-499; Ugurbil, K., Garwood. M., Rath, A. R. and Bendall, M. R. (1988) Amplitude- and frequency/phase-modulated refocusing pulses that induce plane rotations even in the presence of inhomogeneous B.sub.1 fields. J. Magn. Reson. 78:472-497; Ugurbil, K., Garwood, M. and Rath, A. R. (1988) Optimization of modulation functions to improve insensitivity of adiabatic pulses to variations in B.sub.1 magnitude. J. Magn. Reson. 80:448-469) which are based upon the principles of adiabatic passage (Slichter, C. P. In: Principles of Magnetic Resonance, 2nd ed, p. 24, Springer-Verlag, Berlin/New York, 1979) and are insensitive to B.sub.1 inhomogeneity. With these pulses, the need for a separate homogeneous B.sub.1 transmitter coil is obviated, a 250% increase in signal-to-noise is possible, and combined MRI and MRS can be executed in a simple fashion using a single surface coil.
To date, adiabatic pulses which can induce 90.degree. excitation (Bendall, M. R. and Pegg, D. T. (1986) Uniform sample excitation with surface coils for in vivo spectroscopy by adiabatic rapid half passage. J. Magn. Reson. 67:376-381.), non-selective (Baum, J., Tycko, R., Pines, A. (1985) Broadband and adiabatic inversion of a two-level system by phase-modulated pulses. Phys. Rev. A 32:3435-3447; Hardy, C. J., Edelstein, W. A., Vatis, D. (1986) Efficient adiabatic fast passage for NMR population inversion in the presence of radiofrequency field inhomogeneity and frequency offset. J. Magn. Reson. 66:470-482) and selective (Silver, M. S., Joseph, R. I., Hoult, D. I. (1984) Highly selective .pi./2 and .pi. pulse generation. J. Magn. Reson. 59:347-351) inversion, and 90.degree. (Ugurbil, K., Garwood, M. and Bendall, M. R. (1987) Amplitude and frequency modulated pulses to achieve 90.degree. plane rotations with inhomogeneous B.sub.1 fields. J. Magn. Reson. 72:177-185) and 180.degree. (Bendall, M. R., Garwood, M. Ugurbil, K. and Pegg, D. T. (1987) Adiabatic refocusing pulse which compensates for variable RF power and off resonance effects. Magn. Reson. Med. 4:493-499, Ugurbil, K., Garwood, M., Rath, A. R. and Bendall, M. R. (1988) Amplitude- and frequency/phase-modulated refocusing pulses that induce plane rotations enven in the presence of inhomogeneous B.sub.1 fields. J. Magn. Reson. 78:472-497) plane rotations have been developed. With these B.sub.1 -insensitive adiabatic pulses, many sophisticated pulse sequences which require uniform rotation angles can now be applied with an inhomogeneous RF transmitter coil (Garwood, M., Ugurbil, K., Rath, A., Bendall, M. R., Ross, B. D., Mitchell, S. L. and Merkle, H. (1989) Magnetic Resonance Imaging with Adiabatic Pulses Using a Single Surface Coil for RF Transmission and Signal Detection. Magn. Res. Med. 9:25-34).
For many surface coil MRI applications, such as spine and joint imaging, in which multiple image reconstructions in different planes may be necessary to establish a diagnosis, 3-D acquisition holds many advantages. The advent of fast low flip angle pulse schemes such as FLASH (Haase, A., Frahm, J., Matthaei, D., Hanicke, W., and Merboldt, K. D. (1986) Rapid NMR imaging using low flip angle pulses. J. Magn. Reson. 67:258-266) and FISP (Oppelt, A., Graumann, R., Barfuss, H. Fischer, H., Hartle, W. and Schajor, W. (1986) FISP a new fast MRI sequence. Electromedica 54:15-18) has made 3-D acquisition practical. Single surface coil 3-D fast imaging requires a pulse waveform capable of generating an arbitrary uniform flip angle over a wide range in B.sub.1 field strength. The new adiabatic plane rotation pulse of the present invention, hereinafter referred to as BIR-4 (B.sub.1 independent rotation), described below, satisfies these criteria, and is thus well suited to 3-D fast imaging experiments using a single surface coil for RF transmission and signal reception. It has been demonstrated that single surface coil fast imaging can be performed using modified versions of our previously developed adiabatic plane rotation pulses, BIR-1 and BIR-2 (Norris, D. G. and Haase, A. (1989) Variable Excitation Angle AFP Pulses. Magn. Reson. Med. 9:435-440). Relative to these pulses, BIR-4 can induce a plane rotation more accurately with less RF power deposition and is less sensitive to variations in resonance offset produced by chemical shift and/or B.sub.o inhomogeneity.